There was the "what license is that post actually under? Is it 2.5? 3.0? 4.0?" which made things difficult.

There was the "what are the actual attribution requirements that SE has for sites that use its content?" This is a bit of an issue because it's never really clear what those requirements are and what you need to do. It can also hurt SEO because it's duplicated content. Furthermore, codidact leadership had already and enough dealings with SE lawyers and likely wanted to avoid any other.

Lastly, there was the desire to make a philosophical break with SE. The codidact founders didn't want to have anything to do with SE.

Some sites are doing ok. Others stood up but didn't have sufficient involvement to keep them going.

For a counter example, "PhysisOverflow" has an import tool that they use. https://www.physicsoverflow.org/4536/import-queue

Having an imported site that is mostly inactive with activity on that same content is even more disappointing than having a mostly empty site. And active mirroring is a time-consuming process that runs into rate limit issues with an API.